Yeah, WS2019 missing the Essentials role sucks. I'm running WSE2016 and I have no way forward so this will be what I am running until the end of days probably....
But wow, nice setup!
With the HBA card, can you get the HDDs to spin down? I tried with my Dell H310 (some 9210 variant IIRC) but no luck.
I am not exactly sure what you want to accomplish. Do you want duplication and fast reads? You might want to consider using hierarchical pools, something like:
Pool A: 12 x 500GB SSD
Pool B: 2x4TB + 2x2TB + 1x 500GB SSD
Pool C: Pool A + Pool B
I would think that writes go fast (Pool A SSD only, Pool B uses the SSD Cache) and that reads go fast as well as they would read from Pool A effectively (even if the request goes out to Pool C).
The downside is that you can only store about 6TB duplicated.